Power attenuators are utilized in a wide variety of applications one example of which is the dimming of high wattage loads such as lamps used in theatrical, film, television and architectural applications. Prior art power attenuators include resistive attenuators, autotransformers and silicon control rectifier (SCR) dimmers.
Resistive attenuators, although capable of attenuating either AC or DC power while maintaining waveform fidelity, waste a tremendous amount of energy in the form of heat which must be dissipated. Autotransformers alternatively, are capable of attenuating AC power only and are generally quite heavy, making them unsuitable for many applications.
SCR dimmers operate by chopping the input waveform at selected intervals. In this manner, power may be attenuated by varying the interval or duty cycle during which the SCR dimmer is active. Although SCR dimmers do not suffer from the same weight and waste of energy shortcomings as do the resistive and Variac dimmers, the SCR dimmer is capable of attenuating AC power only and must operate in synchronism with the phase of the AC power input.
Many improvements have been made in the SCR dimmer since its introduction, although the fundamental components of the dimmer's power switching stage remain unchanged. For example, the power stage of SCR dimmers utilizes a pair of SCRs and a magnetic inductor or choke.
The magnetic inductor is used to slow the rate of current change produced by the sharp voltage steps of the SCRs. This sharp voltage and subsequent current step in the attached load also produces a large amount of electro-magnetic interference (EMI). EMI is manifested as audible noise emanating from the SCR dimmer, audible lamp filament buzz which reduces filament life, as well as many other undesirable effects on electronic equipment attached to or near the AC power source.
In order to minimize the effects of EMI, larger magnetic inductors have been provided. This adds tremendously to the cost and weight of the dimmer. In addition, EMI interference has also dictated that in many instances where audible noise must be kept to a minimum, SCR dimmers must be located in a separate room, remote from the loads they are to control.
Typically, racks with as many as 192 dimmers are located remotely from the loads. This requires many large high power copper cables connecting each load to the dimmer, as well as a specially cooled room to house the racks of dimmers. An additional drawback is that the resultant configuration of one dimmer per circuit allows for only as many control points as there are circuits.